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.Dd $Mdocdate: January 22 2014 $
.Dt TSLEEP 9
.Os
.Sh NAME
.Nm tsleep ,
.Nm msleep ,
.Nm wakeup ,
.Nm wakeup_n ,
.Nm wakeup_one
.Nd process context sleep and wakeup
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.Ft int
.Fn "tsleep" "void *ident" "int priority" "const char *wmesg" "int timo"
.Ft int
.Fn "msleep" "void *ident" "struct mutex *mtx" "int priority" "const char *wmesg" "int timo"
.Ft void
.Fn "wakeup" "void *ident"
.Ft void
.Fn "wakeup_n" "void *ident" "int count"
.Ft void
.Fn "wakeup_one" "void *ident"
.Sh DESCRIPTION
These functions implement voluntary context switching.
.Fn tsleep
and
.Fn msleep
are used throughout the kernel whenever processing in the current context
cannot continue for any of the following reasons:
.Bl -bullet -offset indent
.It
The current process needs to await the results of a pending I/O operation.
.It
The current process needs resources
.Pq e.g. memory
which are temporarily unavailable.
.It
The current process wants access to data structures which are locked by
other processes.
.El
.Pp
The
.Fn wakeup ,
.Fn wakeup_n ,
and
.Fn wakeup_one
functions are used to notify sleeping processes of possible changes to the
condition that caused them to go to sleep.
Typically, an awakened process will -- after it has acquired a context
again -- retry the action that blocked its operation to see if the
.Dq blocking
condition has cleared.
.Pp
The
.Fn tsleep
function takes the following arguments:
.Bl -tag -width priority
.It Fa ident
An identifier of the
.Dq wait channel
representing the resource for which the current process needs to wait.
This typically is the virtual address of some kernel data structure related
to the resource for which the process is contending.
The same identifier must be used in a call to
.Fn wakeup
to get the process going again.
.Fa ident
should not be
.Dv NULL .
.It Fa priority
The process priority to be used when the process is awakened and put on
the queue of runnable processes.
This mechanism is used to optimize
.Dq throughput
of processes executing in kernel mode.
If the flag
.Dv PCATCH
is OR'ed into
.Fa priority
the process checks for posted signals before and after sleeping.
.It Fa wmesg
A pointer to a character string indicating the reason a process is sleeping.
The kernel does not use the string, but makes it available
.Pq through the process structure field Li p_wmesg
for user level utilities such as
.Xr ps 1 .
.It Fa timo
If non-zero, the process will sleep for at most
.Li timo/hz
seconds.
If this amount of time elapses and no
.Fn wakeup "ident"
has occurred, and no signal
.Pq if Dv PCATCH No was set
was posted,
.Fn tsleep
will return
.Er EWOULDBLOCK .
.El
.Pp
The
.Fn msleep
function behaves just like
.Fn tsleep ,
but takes an additional argument:
.Bl -tag -width priority
.It Fa mtx
A mutex that will be unlocked when the process is safely
on the sleep queue.
The mutex will be relocked at the end of msleep unless the
.Dv PNORELOCK
flag is set in the
.Fa priority
argument.
.El
.Pp
The
.Fn wakeup
function will mark all processes which are currently sleeping on the identifier
.Fa ident
as runnable.
Eventually, each of the processes will resume execution in the kernel
context, causing a return from
.Fn tsleep .
Note that processes returning from sleep should always re-evaluate the
conditions that blocked them, since a call to
.Fn wakeup
merely signals a
.Em possible
change to the blocking conditions.
For example, when two or more processes are waiting for an exclusive lock,
only one of them will succeed in acquiring the lock when it is released.
All others will have to go back to sleep and wait for the next opportunity.
.Pp
The
.Fn wakeup_n
and
.Fn wakeup_one
functions behave similarly to
.Fn wakeup
except that only
.Fa count
or one process, respectively, is marked runnable.
.Sh RETURN VALUES
.Fn tsleep
and
.Fn msleep
return 0 if they return as a result of a
.Fn wakeup .
If they return as a result of a signal, the return value is
.Er ERESTART
if the signal has the
.Dv SA_RESTART
property
.Pq see Xr sigaction 2 ,
and
.Er EINTR
otherwise.
If they return as a result of a timeout, the return value is
.Er EWOULDBLOCK .
.Sh CODE REFERENCES
These functions are implemented in the file
.Pa sys/kern/kern_synch.c .
.Sh SEE ALSO
.Xr hz 9 ,
.Xr mi_switch 9 ,
.Xr timeout 9
